The United Nations officially began a new peacekeeping effort on Monday in the Darfur region of western Sudan in a ceremony that transferred power from the African Union. The Sudanese government had spent months resisting admission of the international force.

The peacekeeping force will be staffed far below the levels set by the United Nations Security Council when it voted unanimously in July to deploy 26,000 troops to try to stop the violence in Darfur. The region has been mired in bloodshed and chaos for four years, as government forces and government-equipped militias have clashed with rebel groups, leaving at least 200,000 civilians dead and 2.5 million villagers homeless.As planned, the peacekeeping force would have been the largest such effort in the world, costing about $2 billion in its first year and drawing on military and police forces from the African Union and the United Nations.

Instead, after months of wrangling, the force that made its debut on Monday was 9,000 strong, United Nations officials said. Experts worry that it may never get to the level where it will make any difference in the region’s troubles.

"It said the UN Security Council received an agreement as far back as June from Sudanese President Omar al-Bashir to accept UNAMID 'without preconditions.'

'And yet the deployment of UNAMID is still facing government obstruction with respect to the force composition and bureaucratic impediments imposed by the Sudanese authorities,' the UN said in a background document. It said the impediments threatened the deployment and detract the hybrid force from its 'potential effectiveness.'

'The persistence of these impediments cannot be attributed to a lack of consultation,' the document said.

The UN has criticized Khartoum for being selective in accepting nationalities of military personnel and deciding which countries should provide the much-needed logistical and transport facilities.

Khartoum had given its preference to African personnel only but has had to accept non-African forces. It rejected troops from Nepal, Nordic engineers and an infantry battalion from Thailand.

The UN has been calling on governments to provide helicopters to assist UNAMID, but so far no country in the world has offered a single helicopter. The United States has offered neither troops nor aircraft, and the UN has not asked for US contributions, knowing that Khartoum would surely object to a US presence in Sudan.

Tension between the African neighbours has risen as the European Union is due to start deploying up to 4,000 troops to Chad and the Central African Republic to help deal with the refugee crisis stemming from Sudan's war-ravaged Darfur region over their eastern border.